1. Field of the Invention
The invention relates to a method for automatic speech direction reversal and a circuit configuration for performing the method.
The convenience of telephone sets has increased markedly in recent years. In addition to automatic redialing, storage in memory of frequently called numbers and speaker phones, the option of speaking without using the handset has become a highly desired telephone feature. Besides its use in the private sector, this feature has many other valuable applications, for instance in interoffice communication, telephone conferences, car phones, or in anywhere else when holding the handset is a problem.
The basic difference between a normal telephone with a handset and a hands-free telephone is in its mode of operation: The normal phone operates in two-way operation. In other words, transmission takes place in both sending and receiving directions simultaneously. In the hands-free telephone, such operation is only possible with very complicated methods, which do not function satisfactorily. Due to the high signal amplification in both directions, any attempt at two-way conversation would immediately produce strong feedback whistling, because a closed loop is created by the acoustic coupling between the loudspeaker and the microphone. Hands-free telephones can therefore operate only in alternating or one-way operation, that is only one of the two participants can ever speak, while the other listens. Attaining such operation requires a circuit that ascertains who is speaking at a given time, in order to then connect the applicable channel and to attenuate the other channel sufficiently. Thus the loop amplification is kept below one. Once the speakers change places, the circuit must ascertain it immediately and switch over the channels accordingly. The circuit operates automatically in the hands-free mode with the aid of a speech direction recognition, combined with an electric reversing switch.
2. Description of the Related Art
A circuit configuration for automatic speech direction reversal in telephones is known, for instance, from the article by Chandra Desai entitled Frei sprechen ohne Ruckkopplung [Hands-free Phoning Without Feedback] Elektronik 22, Oct. 30, 1987, pp. 87 ff., and from Motorola Semiconductors Data Sheet MC 34118.
In that circuit, which is made by analog circuitry, the channel which is not active at a given time is damped relative to the active channel. As a result, the person speaking at a given time cannot hear his partner. A so-called idle state can also be triggered, by operating both channels with half the maximum damping. In the event that neither person is speaking, or if the speaker is drowned out by the noise where the listener is located, then a slow shift to the idle state (slow idle) is made. In contrast, in the event that the speaker and the loudspeaker are at approximately the same level, a fast shift to the idle state (fast idle) is made.
The disadvantages of the known circuit are that with simultaneous speaking and listening (fast idle), the loop amplification can only be kept less than one by means of additional, complicated provisions, and the circuitry is costly as a whole.